This invention relates, generally, to conversion of physical motion into electrical signals, and more particularly to a magnetic pickup for stringed musical instruments which are fretless or which have non-conductive, high resistance frets or non-conductive string wrappings. The invention involves two different methods of generating an audio signal within a single magnetic circuit. The invention can be applied to any fretless metal string instrument having at least two strings. Among such possible applications are the violin, viola, cello, and the double bass. A fretless electric bass is one such instrument for which the invention is well suited.
Variable reluctance pickups of the prior art have become the established method of converting string motion into audio signals; the tonality and "touch response" of the electric bass are due largely to the characteristics of the pickup used. One major disadvantage of prior art variable reluctance pickups is found in those pickups having separate pole pieces or separate magnets dedicated to each string which results in small magnetic fields associated with each string. When the strings are plucked, their motion can exceed the area of the magnetic field, thus causing loss of signal.
When a string is plucked, the player first draws the string out of its restive position, then releases it. Acoustically, the initial attack furnishes a percussive quality and presence. Unfortunately, this initial acoustical vibration cannot typically be converted into an audio signal by conventional variable reluctance pickups. In the large coils of prior art variable reluctance pick-ups the generation of an audio signal in response to the initial attack is delayed due to an opposition to current flow in the coil from the induced electromotive force being generated in the coil.
In prior art variable reluctance pickups, in order to passively drive a length of cable and to match the standard amplifier input impedance, as is required in normal operation of electric stringed instruments, the desired output level is achieved by using a coil having many turns of fine copper wire. The resulting coil has a high impedance. As a result, interference can more easily couple into the pick-up signal path. Additionally, high frequencies are attenuated by the combination of this high impedance and the capacitance and inductance of the coils. Furthermore, the coils often have a resonance within the audio frequency range which causes frequency response peaks in the output signal. These characteristics combine to give a "tonal character" to the pickup. Efforts to reduce the high impedance of the coils, in order to increase their high frequency range or to equalize their response, have required the use of bulky and complex active circuitry in close association with the coils to amplify the signal levels before transmission to a musical instrument amplifier.
Yet, another disadvantage of the prior art variable reluctance pickups is their sensitivity to the proximity of the strings. A 1/32 inch difference in pick-up to string distance can make as much as 30% difference in output level. During the course of playing, these distances will change as the player stops the strings in high and low playing positions. Notes from strings in a high position will appear to leap out at excessive levels while notes from strings in a low position will drop out, all due to the difference in distance of each string from the pickup.
In spite of its disadvantages, however, the variable reluctance pickup is standard on electric basses. Almost all the music played today employs electric basses equipped with variable reluctance pickups. These pickups provide clarity in T.V. and radio broadcasts, as well as in large concert halls. Representative of such variable reluctance pickups are those disclosed in U.S. Pat. Nos. 3,018,682 to Les Paul; 3,035,472 to Freeman; 3,066,567 to Kelley, Jr.; 3,147,332 to Fender; 3,249,677 to Burns, et al.; 3,236,930 to Fender; 3,571,483 to Davidson; 4,069,732 to Moskowitz, et al.; 4,133,243 to DiMarizio; 4,151,776 to Stich; 4,220,069 to Fender; and 4,222,301 to Valdez.
The electric bass has replaced the bass viol in most commercial music application because of its portability, playability, and audibility. However, many listeners, bassists, and arrangers realize the tonal quality of the electric bass is not as pleasing as that of the bass viol. Many electric bassests have switched from the fretted electric bass to the fretless electric bass to regain the expressive qualities only fretless instruments can afford the player. However, these efforts to regain such expressive qualities remain hindered by the limited range of tonal qualities offered by the variable reluctance pickups of the prior art.
Miessner, U.S. Pat. No. 1,915,858; Benioff, U.S. Pat. No. 2,239,985; Valsiach, U.S. Pat. No. 2,293,372; Cookerly, et al., U.S. Pat. Nos. 3,325,579 and 3,297,813; and Moskowitz, et al., U.S. Pat. No. 4,069,732 make use of currents magnetically induced in strings in fretted musical instruments. These configurations are also troubled by uneven string response levels and limited tonal range. In part, these problems were caused by the requirement for electrical return paths routed through the neck of the instrument in order to complete the string transducer circuit. These return paths tend to add additional impedance into the circuit and additionally act as antennas to introduce interference into the signal paths from stray fields. Furthermore, in certain of the prior configurations the interaction between strings within the circuit tended to be in opposition to one another rather than supportive thereof.